In the context of nuclear waste disposal, rock salt has several favorable features, including near-zero permeability in the undisturbed state, very low porosity, and relative high thermal conductivity as compared to other potential host rock alternatives. Moreover, rock salt creeps under deviatoric stresses and temperature changes, and has the capability to heal fractures under favorable stress state. The excavated, run-of-mine salt (crushed salt) can be used as backfill material. As the emplacement drifts close due to the creep of the host rock, the crushed salt backfill undergoes compaction and a reconsolidation Once consolidated, it is expected that the crushed salt will provide an engineered barrier function, thereby reinforcing the geological barrier provided by the natural salt host rock.
Since 2012, LBNL’s work related to Salt R&D has been focused on developing and testing of a numerical modeling tool (TOUGH-FLAC) that can be applied to predict coupled THMC processes associated with nuclear waste disposal in salt formations. The work has a strong component of international collaboration with Clausthal Technical University (TUC) in Germany, a world leading institution on salt geomechanics testing and modeling. LBNL and TUC collaborate on development and application of coupled THMC models, including the development and testing of advanced constitutive models for evaluating the long-term coupled THMC behavior of salt. As part of this effort, LBNL is currently collaborating with Sandia National Laboratories and Los Alamos National Laboratory on field tests being conducted at the Waste Isolation Pilot Plant (WIPP) in New Mexico. LBNL conducts modelingt and geophysical monitoring of the THMC evolution during the experiments.
THM Modeling of field experiment and GDSA input
Research highlight: The TOUGH-FLAC simulator has the capabilities of predicting important coupled processes, including the long-term compaction and permanent sealing of the repository tunnels, as well as large-scale thermal pressurization. The output from such model simulations into the GDSA are the evolution of porosity and permeability within the crushed salt backfill and the disturbed zone. However, there are considerable uncertainties in the input parameters and material properties for such a complex THM model. Field testing and interpretative modeling of such field tests are the most effective methods for reducing the uncertainties in evaluation of parameters and for increasing the understanding of the different processes affecting the long-term behavior of salt host rock exposed to high temperature.
Blanco-Martín L., Rutqvist J., Birkholzer J.T. (2015a) Long-term modelling of the thermal-hydraulic-mechanical response of a generic salt repository for heat-generating nuclear waste. Engineering Geology, 193, 198–211.
Blanco-Martín L., Wolters R., Rutqvist J., Lux K.-H., Birkholzer J.T. (2015b) Comparison of two simulators to investigate thermal-hydraulic-mechanical processes related to nuclear waste isolation in saliniferous formations. Computers and Geotechnics, 66, 219–229.
Blanco-Martín L., Rutqvist J., Birkholzer J.T. (2017) Extension of TOUGH-FLAC to the finite strain framework. Comput. Geosci, 108, 64-71.
Blanco-Martín L., Rutqvist J., Battistelli A., Birkholzer J.T. (2018) Coupled Processes Modeling in Rock Salt and Crushed Salt Including Halite Solubility Constraints: Application to Disposal of Heat-Generating Nuclear Waste. Transp Porous Med 124:159–182.
Contact: Jonny Rutqvist
Field monitoring and supporting laboratory tests
Research highlight: We are conducting borehole heater tests at the Waste Isolation Pilot Plant (WIPP) called the Brine Availability Test in Salt (BATS). The main focus of the BATS field test is on brine availability in salt, and these field tests are the first part of a wider systematic field investigation campaign to improve the existing long-term repository safety case for disposal of heat-generating radioactive waste in salt. As part of the team, LBL is conducting geophysical monitoring with a focus on electrical resistivity tomography (ERT) and distributed fiber optic temperature and strain sensing.
The layout of the test location is shown in the figure below.
Selected publications: N/A
Contact: Yuxin Wu
International modeling efforts including BenVaSim
Research highlight: LBNL has under close collaboration TUC Germany, developed and applied the TOUGH-FLAC simulator for nuclear waste disposal in salt formations. This includes the advanced Lux-Wolters salt THM constitutive model that can handle permeability changes due to mechanical damage under high deviatoric stress and subsequent healing. In collaboration with TUC, LBNL has used data Thermal Simulation for Drift Emplacement (TSDE) test in Asse Mine, Germany, for model validation, in particular related to creep compaction of the crushed backfill under high temperature. LBNL is also an active participant within the international BenVaSim code comparison project and will participate in a new task in the international model comparison project DECOVALEX-2023, in which BATS will be analyzed in collaboration with over ten international research teams. Related to the DECOVALEX-2023, LBNL is part of a DEO funded research team in collaboration with Sandia National Laboratories and Los Alamos National Laboratory.
Blanco-Martín L., Wolters R., Rutqvist J., Lux K.-H., Birkholzer J.T. (2016) Thermal–hydraulic–mechanical modeling of a large-scale heater test to investigate rock salt and crushed salt behavior under repository conditions for heat-generating nuclear waste. Computers and Geotechnics, 77, 120–133.
Guiltinan E.J., Kuhlman K.L., Rutqvist J., Hu M., Boukhalfa H., Mills M., Otto S., Weaver D.J. Dozier B., Stauffer P.H. 2020. Temperature response and brine availability to heated boreholes in bedded salt. Vadose Zone J. 19:e20019 (2020). https://doi.org/10.1002/vzj2.20019
Contact: Jonny Rutqvist